Circulating current battery heater

ABSTRACT

A circuit for heating energy storage devices such as batteries is provided. The circuit includes a pair of switches connected in a half-bridge configuration. Unidirectional current conduction devices are connected in parallel with each switch. A series resonant element for storing energy is connected from the energy storage device to the pair of switches. An energy storage device for intermediate storage of energy is connected in a loop with the series resonant element and one of the switches. The energy storage device which is being heated is connected in a loop with the series resonant element and the other switch. Energy from the heated energy storage device is transferred to the switched network and then recirculated back to the battery. The flow of energy through the battery causes internal power dissipation due to electrical to chemical conversion inefficiencies. The dissipated power causes the internal temperature of the battery to increase. Higher internal temperatures expand the cold temperature operating range and energy capacity utilization of the battery. As disclosed, either fixed frequency or variable frequency modulation schemes may be used to control the network.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of United States patent application Ser.No. 09/070,331, filed Apr. 30, 1998, now U.S. Pat. No. 5,990,661.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under NREL subcontractnumber ZAN-6-16334-01, prime contract number DE-AC36-83CH10093 issued bythe Department of Energy. The government has certain rights in thisinvention.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates generally to devices for heatingbatteries. More particularly, the invention relates to electricalcircuits for using the stored energy of a battery to heat either theconnected battery or a peripheral battery. The invention also relates toelectrical circuits for transferring energy between a plurality ofseries connected batteries and equalizing the energy within thebatteries.

With the current growth in electrical and electronic technology, thereis a growing interest in using batteries as a primary source of power,for backup applications, and for starter, lighting, ignition (SLI)applications. Electric vehicles, hybrid vehicles, military electronicsystems, consumer vehicles, communications systems, medical emergencyequipment, and handheld power tools are among the more promisingapplications requiring batteries.

Batteries provide a uniquely portable source of energy that is notdependent on a connection to a power grid. However, the operation of abattery is limited by a number of factors, one of which is temperature.At low temperatures the capacity of a battery to store energy, themaximum value of discharge current which can be drawn from the battery,and the cold cranking amps capability decrease substantially. Manyapplications require batteries either as a primary power source, suchas; electric vehicles, hybrid vehicles, and handheld power tools, or as;a backup power source such as communications systems, medical emergencyequipment, and military electronic systems, or also as; an SLI sourcesuch as commercial vehicles. Batteries used for SLI applications at coldtemperatures additionally must be capable of supplying greater amountsof cold cranking current to overcome the increased engine resistancecaused by decreased engine oil viscosity at low temperatures. Most ifnot all of these devices are exposed to outdoor environments andtherefor must be able to operate at low temperatures.

Current methods of operating batteries at low temperatures employ meansfor heating the battery from an external source, such as warm airheating, liquid heating, and thermal jackets. Conventional batteryheating systems typically use a separate power source to power a heatingelement which generates the required heat. The resultant heat is thentransferred to the battery by either a convection system or a conductionsystem. Convection systems blow hot air across the battery, whereasconduction systems apply heat directly to the surface of the battery.Each of these systems warm the battery by heating the external surfaceof the battery. By applying heat to the external surface a significantamount of the generated heat is lost to the external environment. Bothconvection and conduction systems additionally require some form ofmechanical structure co-located with the battery. Also, warm air heatingand liquid heating require complex mechanical systems that usesubstantial amounts of external power. These constraints limit theportability of batteries and demand the availability of an externalpower source.

A system employing thermal jackets typically includes a flexibleinsulator that wraps around the exterior of a battery. On the innersurface of the insulator is an externally powered heating element. Whilethermal jackets do not require a complex mechanical system, extra spacearound the individual batteries must be set aside. Additionally, thethermal jacket systems known in the art must be powered by a separatepower source or power grid. While heating systems and thermal jacketscan be used to extend the ambient temperature range within whichbatteries can be operated, they have not proven capable of efficientlyheating a battery without an external power source and bulky externalmechanical attachments.

Accordingly, it is desirable to overcome the disadvantages associatedwith the prior art systems. The present invention addresses this problemby circulating energy from within the battery into lossless or nearlylossless energy storage devices and then back into the battery causingthe battery to dissipate power internally due to theelectrical-to-chemical energy conversion losses and conduction losses ofthe battery. The internally dissipated power heats the battery, causingthe internal temperature to rise. A higher internal temperatureincreases the operating temperature range, improves the cold crankingamps capability, and improves the energy capacity utilization of thebattery. The circuit of the present invention does not require a complexmechanical structure, and efficiently heats the battery from the insideout. The circuit enhances the portability of batteries into cooleroperating environments by using the internal energy of the battery forself-heating. The circuit takes energy from the battery and stores it inan essentially lossless element and then recirculates that energy backinto the battery. In transferring energy first out of the battery andthen back into the battery some losses will occur within the battery dueto inefficiencies in converting electrical energy into chemical energy.The losses result in increased heat within the battery thereby causingthe battery to heat from the inside out. Additionally, there are veryfew constraints on the location of the circuit relative to the battery.It is possible to place the circuit a significant distance away from thebattery, depending on the magnitude of the circulating current.Additionally the circuit of the present invention allows the energypotential within the individual cells of an energy storage device to beequalized.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, its objects andadvantages, reference may be had to the following specification and tothe accompanying drawings in which:

FIG. 1 is a schematic diagram showing a preferred embodiment of theinvention;

FIG. 2 is a signal diagram showing the current and voltage waveformsassociated with a preferred embodiment of the present invention;

FIG. 3 is a schematic diagram showing an alternate preferred embodimentof the invention used for battery voltage equalization; and

FIG. 4 is an isolated schematic diagram of the switching circuit used inaccordance with the preferred embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an exemplary circulating current battery heatercircuit 10 according to the teachings of the present invention is shown.The present implementation of circuit 10 is a variable frequencyhalf-bridge configuration operating at a frequency less than 25 Khz. Thecircuit 10 heats a string of Li-Ion batteries 12, 13 having a combinedvoltage of 144 volts. Other types of batteries including but not limitedto; lead-acid, NiCad, NiMH, alkaline, and lithium batteries are withinthe scope of the present invention. Additionally, either one or morebatteries 12, 13 may be heated, such as batteries units ranging from a12 volt car starter battery to 600 volt battery assemblies for electriccars or buses. Although the preferred embodiment employs a variablefrequency modulation (FM) technique, the principles of the invention mayequally be extended to the constant frequency pulse-width modulationtechnique. The circuit 10 should be operated over a frequency range thatminimizes the inductor weight and size without creating excessiveoverall losses in the inductor core, coil, and switching losses of thepower semiconductor switches.

The presently preferred embodiment includes a pair of Insulated GateBipolar Transistors (IGBTs) 14 and 16 connected in a half-bridgeconfiguration. However, the principles of the invention may be extendedto the use of other switching devices such as; MOSFET's, BJT's, andMCT's. Anti-parallel diodes 18 and 20 are connected in parallel withIGBTs 14 and 16 respectively. Inductor 22 is connected from the junction30 of IGBTs 14 and 16 to the junction 32 of the energy storage means (orupper string of batteries) 13 and the lower string of batteries 12.Although an inductor is used for the series resonant element in thepreferred embodiment, a series LC combination is also within the scopeof the invention.

The energy storage means 13 in the present embodiment is preferably astring of batteries equivalent in voltage to the lower string ofbatteries 12. However, the energy storage means may also include otherenergy storage devices such as various types of capacitors including;ceramic, MLC, plastic film, plastic foil, oil-filled, aluminumelectrolytic, and tantalum electrolytic capacitors. A voltage monitor 24is provided for sensing voltage across each string of batteries 12 and13. A temperature monitor control 26 is provided for sensing thetemperature of each of the batteries via temperature sensors 34 and 36associated with each string of batteries 12 and 13. The invention alsoencompasses potentially monitoring the circulating current and charge inbatteries 12 and 13. Current monitoring may be accomplished by use of aHall effect circuit or a resistive shunt in series with inductor 22.Driving an integrator circuit in a feedback loop with a sensed currentsignal would provide battery charge and current monitoring and control.Both current and charge monitor and control circuits are well known inthe art. The outputs of the voltage monitor 24 and the temperaturemonitor control 26 connect to FM driver 28 which may also function asthe system controller. A battery undervoltage or overtemperaturecondition will be sensed by the monitor circuits 24, 26 which willdisable FM driver 28, thereby protecting the battery from a potentiallydestructive operating condition. The two outputs of FM driver 28 connectfrom gate to emitter on each of the IGBTs 14 and 16.

In operation, the preferred embodiment employs frequency modulation withvariable frequency with a maximum value of 25 kilohertz. However, theprinciples of the invention may be readily extended to a circuitemploying pulse-width modulation. As is known in the art, pulse-widthmodulators control a circuit by operating at a fixed frequency andvarying the pulse-width in response to a controlling input. A frequencymodulator controls a circuit by varying the total period of the signal.

Referring to FIGS. 1 and 2, the V_(G1) output from FM driver 28 drivestransistor Q1 14 into the ON state. Voltage from energy storage means 13is impressed across inductor 22 causing current i_(L) to linearlyincrease with time, storing energy within inductor 22. At time t_(ON)(FIG. 2) FM driver 28 drives the V_(G1) signal into the low state. As Q114 begins to turn off the current which flows through inductor 22 willbegin to conduct through lower battery 12 and up through anti-paralleldiode 20. Voltage from the batteries 12 is impressed across inductor 22causing current i_(L) to linearly decrease. During this mode ofoperation energy is transferred from inductor 22 to lower string ofbatteries 12.

As energy is either transferred into or out of the strings of batteries12 and 13, conduction losses and electrical-to-chemical conversioninefficiencies cause power to be dissipated within the batteries. Thedissipated power causes the internal temperature of the batteries torise resulting in very efficient self-heating of the batteries.

At time t_(x) the FM driver 28 drives the V_(G2) output high, turning ontransistor Q2 16. Turning on Q2 16 while current continues to flowthrough anti-parallel diode 20 results in virtually zero turn-on lossesin Q2 16. The turn-on losses of an IGBT are directly proportional to thesquare of the voltage from collector to emitter. Normally, with mostmodulation schemes the voltage from collector to emitter at turn-onwould be approximately 72 volts in this circuit configuration. However,by turning on IGBT 16 while current is flowing in anti-parallel diode20, the voltage impressed from collector to emitter at turn-on isapproximately one diode drop. A diode drop for a high voltage diode isapproximately one or two volts. Therefore, turn-on losses are decreasedby more than a factor of 5,000.

Current in anti-parallel diode 20 continues to decrease until it reacheszero amps, at which time i_(L) begins to flow into the collector of Q216 and up through lower batteries 12. As current flows from batteries 12through inductor 22 energy is transferred from the string of batteries12 into inductor 22 until Q2 16 turns off. At time T the FM driver 28drives the V_(G2) output to the low state turning off Q2 16. As Q2 16turns off, the current flowing through inductor 22 begins to flow upthrough anti-parallel diode 18 and down through the upper string ofbatteries 13. The voltage from batteries 13 is impressed across inductor22 causing current i_(L) to begin to linearly increase. During thisperiod, energy is transferred from inductor 22 into upper string ofbatteries 13. At some time prior to inductor current i_(L) reachingzero, Q1 14 is turned on, thereby attaining virtually lossless turn-onin a manner similar to transistor Q2 16. When current through inductor22 reaches zero amps, anti-parallel diode 18 becomes back-biased and thecurrent once again begins to flow through the loop comprising IGBT 14,inductor 22, and upper string of batteries 13.

Turning now to FIGS. 3 and 4, equalization circuit 40 is disclosed inaccordance with an alternate preferred embodiment of the presentinvention. As shown, equalization circuit 40 comprises four switchingmodules M1, M2, M3 and M4 which are connected to a battery pack 42. Thebattery pack 42 includes five series connected energy storage cells B1,B2, B3, B4 and B5. While battery pack 42 is shown to include five cells,one skilled in the art will appreciate that equalization circuit 40 canbe used with any type of battery pack having a plurality of cells. Eachswitching module is substantially similar to the switching components ofcircuit 10 shown in FIG. 1. Each switching module M1, M2, M3, M4includes a first switch S1 44 connected generally across a first storagecell such as cell B1, and a second switch S2 46 is connected generallyacross a second storage cell such as cell B2. As shown, each switchingmodule overlaps a storage cell with another switching module. In thetypical configuration, the number of switching modules will be one lessthan the number of energy storage cells.

An inductor 50 or similar type of temporary energy storage device isconnected to a node 48 between storage cells B1 and B2, and connected toa node 52 between first and second switches 44, 46 as shown. Inductor 50can be any type of inductor suited for the particular application.Within each switching module, a suitable driver circuit 54 is connectedto first switch 44 and connected to second switch 46. The driver circuit54 may be any of the driver circuits described above with respect toFIG. 1. While the components of switching module M1 are described indetail, the components of the other switching modules M2, M3 and M4 areidentical to switching module M1.

The components forming first switch 44 are shown in detail in FIG. 4. Itshould be understood that second switch 46 includes components which aresubstantially similar to the components of first switch 44. Morespecifically, each first and second switch 44, 46 includes a transistor56, and an anti-parallel diode 58 connected in parallel with thetransistor 56. The cathode of diode 58 is connected to the transistordrain 62, and the anode of diode 58 is connected to the transistorsource 64. The gate 60 and source 64 of transistor 56 are connected tothe driver circuit 54. Preferably, transistor 56 is a field effecttransistor (FET). However, one skilled in the art will readilyappreciate that a variety of transistors may be employed depending uponthe specific application for equalization circuit 40. The switchingmodules M1-M4 shown in FIG. 3 are identical to the circuit external tothe batteries, shown in FIG. 1, except that the voltage monitor 24 andthe temperature monitor 26 have been omitted, and the IGBT's have beenreplaced with FETs because the voltages associated with battery pack 42are preferably lower.

In operation, each switching module M1, M2, M3, M4 is responsible fortransferring energy between adjacent storage cells. For exampleswitching module M1 transfers energy between storage cells B1 and B2,and switching module M2 transfers energy between storage cells B2 andB3. The remaining switching modules M3, M4 operate in a similar fashionwith storage cells B3-B5. As a result, the purpose of switching modulesM1-M4 is to equalize the energy potentials or voltages of battery cellsB1-B5.

As part of normal operation, the voltages V₁-V₅ tend to becomeunbalanced during charge and discharge due to slight variations amongthe individual storage cells B1-B5. Since the batteries B1-B5 have highand low voltage limits, the highest will limit charging, and the lowestwill limit discharge. Maximum charge capacity is achieved when thevoltages of each of the batteries B1-B5 are equal.

As part of the operational example presented herein, assume that thevoltage V₂ of battery B2 is lower than the voltage V₁ of battery B1.Switching module M1 will transfer charge from B1 to B2 as follows. Whentransistor Q1 56 (of switch S1) is turned on by driver circuit 54 andconducts, energy from battery B1=E₁=0.5L₁I_(Q1) ² (peak) is removed frombattery B1 and stored within the inductor 50. When transistor Q1 (ofswitch S1) is turned off by driver circuit 54, this energy istransferred to battery B2 via diode D2 (of switch S2). Transistor Q2 (ofswitch S2) is then turned on by driver circuit 54 and conducts energy ina similar fashion for storing energy from battery B2 within the inductor50. After transistor Q2 is turned off by driver circuit 54 and finishesconducting, energy from battery B2=E₂=0.5L₁I_(Q2) ² (peak) istransferred back to battery B1 via diode D1 58 (of switch S1). However,since V₁>V₂, this means I_(Q1(peak))>I_(Q2(peak)) and E₁>E₂, i.e. thereis a net energy transfer, E₁−E₂ from battery B1 to battery B2. This alsomeans that on each cycle there will be a net charge transfer from B1 toB2 until the voltages V₁ and V₂ become equal.

In a similar manner switching module M2 equalizes batteries B2 and B3;switching module M3 equalizes batteries B3 and B4; and switching moduleM4 equalizes batteries B4 and B5. No matter how the initial voltageimbalances are distributed, energy will be transferred between batteriesuntil all voltages are equal. When complete equalization is realized,the driver circuits 54 can be turned off.

More efficient strategies can be used if a microprocessor, or any typeof computer (not shown) is implemented to measure the voltages V₁-V₅ andto control the switches S1 44 and S2 46 in modules M1-M4. For example,if the microprocessor determines that V₁ is the highest voltage and V₃is the lowest voltage, the switches can transfer energy from V₁ to V₃ asfollows.

1. Close the S1 transistor of switching module M1 to transfer energyfrom battery B1 to inductor L1 50.

2. Open the S1 transistor of switching module M1 to transfer energy frominductor L1 to battery B2 via the diode in S2 (the S2 transistor remainsoff).

3. Close the S1 transistor of switching module M2 to transfer energy tofrom battery B2 to inductor L2.

4. Open the S1 transistor of switching module M2 to transfer energy fromthe inductor L2 to battery B3 via the diode in S2 of switching moduleM2.

The operation is similar for any pair of highest and lowest voltages,and this pair may change as operation proceeds. The microprocessorcontinuously changes the switch operations so that energy is alwaysbeing transferred from the present highest to the present lowestvoltage. This second method is more efficient because energy istransferred in only one direction, whereas in the first method a largepart of the energy was first removed from each battery and then returnedto the same battery.

From the foregoing it will be understood that the invention provides acircuit that employs a low loss network for using the internal energy ofa battery to heat either the same battery or another battery. Theinvention also provides a circuit for equalizing the voltage potentialof the cells within a battery pack. The circuit may be readily scaled byselection of appropriate switching devices and energy storage devices toaccommodate different quantities of batteries, different types ofbatteries, and different methods of storing the intermediate energy. Thecircuit thus provides a novel and efficient means for heating batteriesin a wide range of applications.

Through the use of a low loss switched energy storage network, theinvention can provide a means of increasing the effective operatingtemperature range of batteries and improving the utilization of theenergy stored within batteries operating within low temperatureenvironments. Additionally the invention improves the low temperaturecharging efficiency of a battery. The circuit enhances the portabilityof systems that employ batteries by providing means for using energystored within the battery to self heat the battery. The invention istherefore ideally suited for heating electric vehicle batteryassemblies, as well as batteries used with hand held power tools. Thecircuit will heat both multiple batteries and single batteries usingeither variable frequency modulation or pulse-width modulation.

While the invention has been described in its presently preferredembodiment, it will be understood that the invention is capable ofcertain modifications and change without departing from the spirit andscope of the invention as set forth in appended claims.

What is claimed is:
 1. A circuit for heating an energy storage devicecomprising: a first switching device connected to the energy storagedevice; a second switching device connected to the energy storagedevice; and an inductor disposed between the first and second switchingdevices; wherein the first and second switching devices each include atransistor connected to the energy storage device and connected to theinductor and the first and second switching devices each include a diodeconnected in parallel with the transistor.
 2. A device for circulatingenergy for performing a heating function comprising: an energy storagedevice comprising a plurality of energy storage cells; a first switchingcircuit connected to the energy storage device; a second switchingcircuit connected to the energy storage device; and an inductorconnected between the first and second switching circuits; wherein thefirst and second switching circuits are alternately switched between acurrent conducting state and a non-conducting state for transferringenergy back and forth between the energy device and the inductor;wherein the first and second switching circuits each include atransistor connected to the energy storage device and connected to theinductor and the first and second switching circuits each include adiode connected in parallel with the transistor.
 3. A device forequalizing the energy potential within the cells of an energy storagedevice comprising: at least a first storage cell and a second storagecell contained within the energy storage device; a first switchingcircuit connected across the first storage cell; a second switchingcircuit connected across the second storage cell; and an inductorconnected between the first and second switching circuits; the first andsecond switching circuits being alternately switched between a currentconducting state and a non-conducting state for transferring energybetween the first storage cell, the inductor, and the second storagecell, wherein the first and second switching circuits are alternatelyswitched until a net energy transferred between the first storage celland the second storage cell approaches zero; wherein the first andsecond switching circuits each include a transistor connected to theenergy storage device and connected to the inductor and the first andsecond switching circuits each include a diode connected in parallelwith the transistor.
 4. A circuit for heating an energy storage devicecomprising: a first switching device connected to the energy storagedevice; a second switching device connected to the energy storagedevice; an inductor disposed between the first and second switchingdevices; and a frequency modulation circuit for alternately switchingthe first and second switching devices between a conducting state and anon-conducting state.
 5. A device for circulating energy for performinga heating function comprising: an energy storage device comprising aplurality of energy storage cells; a first switching circuit connectedto the energy storage device; a second switching circuit connected tothe energy storage device; and an inductor connected between the firstand second switching circuits; wherein the first and second switchingcircuits are alternately switched between a current conducting state anda non-conducting state for transferring energy back and forth betweenthe energy device and the inductor; and a frequency modulation circuitfor alternately switching the first and second switching circuitsbetween a conducting state and a non-conducting state.
 6. A device forequalizing the energy potential within the cells of an energy storagedevice comprising: at least a first storage cell and a second storagecell contained within the energy storage device; a first switchingcircuit connected across the first storage cell; a second switchingcircuit connected across the second storage cell; and an inductorconnected between the first and second switching circuits; the first andsecond switching circuits being alternately switched between a currentconducting state and a non-conducting state for transferring energybetween the first storage cell, the inductor, and the second storagecell, wherein the first and second switching circuits are alternatelyswitched until a net energy transferred between the first storage celland the second storage cell approaches zero; a frequency modulationcircuit for alternately switching the first and second switchingcircuits between a conducting state and a non-conducting state.